High performance materials such as graphite/epoxy composites allow the production of extremely strong, stiff, yet lightweight structures. However, there is a lack of data on their fatigue and failure modes, since these materials have not yet been in service long enough. This lack of statistical databases has limited their widespread acceptance. Furthermore, structures using these new composites tend to be over designed, which, in turn, may reduce or eliminate the benefits of using these materials. A related paper is by Spillman, W. B., Durkee, S.; Non-Contact Interrogation System for Smart Structures, Proceedings of SPIE vol. 2191, paper no.43, Proc. Smart Sensors, Processing and Instrumentation Session, North American.
Structural health monitoring would allow the realization of the materials full potential, since structures could be designed to their material limits. Embedded sensors could communicate information such as strain, stress, temperature, and pressure, to allow in service tracking of the materials behavior over time. Changes in behavior would indicate the need for service, repair, or replacement of component(s). This would increase our understanding of the characteristics of advanced materials, as well as improve our safety when these new structures are used in critical applications.
Recent efforts to produce structural monitoring systems have focused on integration of the sensors into the new materials. Related papers are in Proceedings of the 1st European Conference on Smart Structures and Materials, Glasgow, 1992 and SPIE Vol. 1798, Proceedings of the Fiber Optic Smart Structures and Skins V Conference, 1992. Embedded fiber optic sensors, capable of stress/strain measurements over large areas, have been developed, but the problem of connecting these fiber networks to an interrogation system still needs to be addressed. Machining and trimming of composite parts after initial fabrication can damage or destroy direct, contacting type connections to embedded sensors.
Prior patented art in the medical field includes that of Hogrefe et al. (U.S. Pat. No. 4,561,443; issued Dec. 31, 1985) which describes a coherent inductive communications link for biomedical applications. This two way inductive link provided communications between an external transceiver and an internal transceiver located in a biologically implantable medical device. Digitally formatted command data and programming data was transmitted to the implanted medical device by frequency shift keying in inductive communications link. The internal tranceiver was powered by the inductive field between the internal and external transceivers. Digitally formated data was transmitted to the external transceiver by internal transceiver amplitude modulating field. This patent describes means for communicating with an implanted (or embedded) device, but it does not describe means for conditioning or utilizing sensors or addressing sensor networks.
The development of improved implantable devices and materials requires knowledge of their in vivo behavior. However, little is known about the actual loads borne by implanted devices in vivo. Direct load measurement would provide extremely valuable information, for the improvement of device designs, and for the rapid rehabilitation of individuals in which devices have been implanted. Multichannel telemetry systems, combined with strain gauges, can provide this information.
Previous authors have developed and applied implantable telemetry for strain gauged implants. See Bergmann et al. xe2x80x9cMultichannel Strain Gauge Telemetry for Orthopedic Implants. xe2x80x9d J. Biomech., Vol 21, No. 2, Pages 169-176, 1988 and Rohlmann et al., xe2x80x9cTelemeterized Load Measurement using Instrumented Spinal Internal Fixators in a Patient with Degenerative Instabilityxe2x80x9d, Spine, Vol 20, No 24, pages 2683-2689, 1995. However, these systems did not possess the capability of software programmability. By combining advanced, micropower, analog/digital integrated circuits (IC) with new, miniature, low power microprocessors, more versatile implantable telemetry systems can be realized, as described in C. P. Townsend and S. W. Arms, xe2x80x9cMultichannel, Programmable, Microprocessor Based Strain Gauge Telemetry Systemxe2x80x9d, Presented at 18th Ann. Int""l Conf. IEEE Eng. in Medicine and Biology Soc. Oct. 31-Nov. 3, 1996, Amsterdam, The Netherlands.
Prior patented art in the field of smart structures includes that of Spillman et al. (U.S. Pat. Nos. 5,440,300, 5,581,248, and 5,703,576). These patents describe a non-contact power and data interface for smart structures, an embeddable device for contactless interrogation of sensors, and an embeddable DC power supply for smart structure sensors. However, they also do not describe techniques for addressing networks of embedded, multichannel sensing nodes; this limits their utility when multiple sensors are required. These patents also do not make reference to the use of an embedded (remote) microprocessor, and do not use digital encoding methods for conversion of sensor signals and for data communications.
However, the use of a microprocessor provides distinct advantages, as it facilitates digital conversion and communications,and improves data fidelity. Furthermore, an embedded microprocessor can be used to send error checking information (checksum byte(s)) from the remote (or embedded) systems to the external interrogation unit. This allows the external unit to detect errors in (wireless) data transmission, and to xe2x80x9cflagxe2x80x9d those data that may have been altered by interference.
Development of non contact power/interrogation systems for use with embedded sensing networks would eliminate direct connections and solve the problems associated with them, as described in a paper by Townsend, C. P. and Arms, S. W., xe2x80x9cMethod for Remote Powering and Communication with an Embedded Network of Addressable Sensing Modulesxe2x80x9d, SPIE""s 4th Annual Symposium on Smart Structures and Materials, Mar. 3-6, 1997, San Diego, Calif.
Smart structures should not be limited to specific sensing means, such as fiber optic type, since other sensing technologies can provide very useful information. Examples of these include: foil and piezo-resistive strain gauges, inductive and capacitive sensors, temperature probes, accelerometers, inclinometers, and magnetometers. Recent advances in transducer signal conditioners have produced very small, fully integrated, linear circuits for use with many of these devices. Related information is in Analog Devices, Monolithic Instrumentation Amplifier, part no. AD620, Amplifier reference manual, Norwood, Mass., 1992, Linear Technologies, Precision, Micropower, Instrumentation Amplifier, part no. LT1101, Linear Databook, Milipitas, Calif., 1990, and Analog Devices, LVDT Signal Conditioner, part nos. AD598 and AD698, Special Linear Reference Manual, section 10-23, Norwood, Mass., 1992. Intelligent, addressable sensing modules, with built-in signal conditioning and data transmission capabilities, are needed to provide data in a common (digital) format. These modules can then be used to create generalized sensing networks.
It is the object of this invention to teach a remote powering and communications method, combined with multichannel, microprocessor based sensing modules, especially for use with advanced sensate medical implants. It is further the object of this invention to describe the combination of embedded microproccessors, highly integrated sensor signal conditioners, digital data converters, and the use of networking techniques, especially for smart structure applications, in order to overcome the limitations of previous non contacting power links and analog data transmission systems.
It is the object of this invention to teach a multidrop network of multichannel, addressable sensing modules (ASM""s), to be embedded within a composite structure, remotely powered, and interrogated by a personal computer through a non-contacting inductive link. Each ASM contains a microprocessor with non-volatile memory, multiplexer, programmable gain and filter instrumentation amplifier, and sigma delta analog to digital converter (all housed in two thin surface mount packages). An embedded mothernode includes circuitry for power and data reception (into the structure), and data transmission (back out of the structure).
It is the object of this invention to teach an external interrogation system which communicates into the network of ASM""s by modulating the AC waveform that delivers power to the embedded electronics. Once addressed, each ASM powers up its programmable (gain and filter) sensing channels (3 full differential or 5 pseudo differential). and data conversion elements. Sensed data are pulse code modulated, including error checking, which serially modulate an RF carrier for wireless transmission out of the composite to the interrogating computer.
These networks will allow smart structures to be designed with a broad variety of capabilities. For example, temperature and strain information would be very useful during composite materials fabrication process. Exposure to cyclic strain, stress, pressure, electromagnetic fields, or radiation could be of interest in service. An open network architecture permits the addition or subtraction of various sensing modules, in order to best meet the requirements of a specific application. This capability can provide low cost structural health monitoring, as well as security, intelligence gathering, and control functions to be implemented as needed.